1. Field of the Invention
The present invention relates to an endoscope insertion unit inserted into a body cavity, an endoscope, and an endoscope system.
2. Description of the Related Art
Endoscopes have come to be widely used in recent years which have an image capturing unit composed of, for example, an object optical system, solid-state image capturing device and circuit board on a distal end of an endoscope insertion unit. The use of this type of endoscope enables a physician to perform various procedures such as observation and treatment of internal organs.
In the case of inserting this type of endoscope into a body cavity, it may be difficult for the physician to insert the endoscope into the body cavity due to an action which tends to contract the body cavity. It is particularly difficult to insert an endoscope insertion unit into a curved portion of a body cavity, such as the sigmoid colon of the large intestine.
The endoscope insertion unit preferably has lower flexibility on the proximal side than the distal side in order to enhance the ease of inserting the insertion unit into a curved portion of a body such as the sigmoid colon of the large intestine. In other words, having higher flexibility for the flexible portion on the distal end of the endoscope insertion unit than the flexibility for the flexible portion on the proximal end makes it easer for the insertion unit to follow the curvature of a curved portion, thereby improving the ease of insertion into a curved portion such as the sigmoid colon.
Consequently, after the distal end of the flexible portion of an endoscope insertion unit has passed a curved section such as the sigmoid colon, the lower the flexibility of the proximal end of the flexible portion of the insertion unit, the less susceptibility to the effect of the force which tends to contract the large intestine. Thus, a physician is able to insert the endoscope insertion unit into the large intestine while smoothly pushing the insertion unit into deeper sections of the large intestine.
Therefore, an endoscope is disclosed in, for example, Japanese Patent Application Laid-open No. 2001-190494, in which, by forming the wall thickness of the outer sheath of a flexible tube of an endoscope to gradually become thicker moving from the distal end to the proximal end, a portion of the flexible tube closer to the proximal end has lower flexibility than the portion closer to the distal end.
In addition, there is a technology for facilitating insertion of an endoscope into a curved section such as the sigmoid colon by using an endoscope insertion assisting device in the form of a so-called endoscope overtube. By inserting this endoscope overtube into a body cavity together with an endoscope, for example, an insertion path for the endoscope is secured within the body cavity, thereby facilitating subsequent insertion and removal of the endoscope. An endoscope system using this type of endoscope overtube is described in, for example, Japanese Patent Application Laid-open No. 2002-369791.